Two-part structural adhesive systems and laminates incorporating the same

ABSTRACT

A two-part adhesive systems exhibiting improved fixture and open times are disclosed. In one embodiment, the system includes (a) an adhesive part being a mixture formed from (I) an ethylenically unsaturated monomer, (II) a metal molybdate, (III) a metal salt of an ethylenically unsaturated carboxylic acid in an amount less than 3 weight percent, and (IV) an ethylenically unsaturated carboxylic acid, and (b) an activator part which includes a free radical generator. In another embodiment, the system includes the above-described adhesive part substantially free of a metal salt of an ethylenically unsaturated carboxylic acid. Curable adhesives and laminates prepared with the two-part system are also disclosed.

RELATED APPLICATIONS

This application is a division of application Ser. No. 10/072,696, filedFeb. 7, 2002 now U.S. Pat. No. 6,730,411 entitled “TWO-PART STRUCTURALADHESIVE SYSTEMS AND LAMINATES INCORPORATING THE SAME”, which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to two-part structural adhesives usefulfor bonding metal and polymeric substrates and more particularly toacrylate and methacrylate-based two-part structural adhesives andcoatings.

BACKGROUND OF THE INVENTION

Structural adhesives have found broad use in the modern-daymanufacturing environment. In the automobile industry, for example, theadhesive bonding of metals to plastics has found wide application wherethe use of rivets or other mechanical fasteners is impractical due toconsiderations of aesthetics, cost or corrosion (due to the use ofdissimilar metals). Such adhesive bonding applications have foundbroader acceptance due to recent advancements in formulations ofadhesives, which allow effective bonding to metals (see U.S. Pat. Nos.4,293,665 and 4,452,944, both assigned to Lord Corp.), bonding tosolvent-resistant plastics (see U.S. Pat. No. 4,714,730, assigned toITW, Inc.) and for low temperature applications (see U.S. Pat. No.4,942,201, assigned to ITW, Inc.). The teachings of these fourreferences are hereby incorporated by reference.

The effectiveness and strength of these adhesives allows for thesubstitution of adhesives for mechanical connectors such as bolts andrivets. In metal-bonding applications, such mechanical connectors canhave disadvantages, such as appearance, cost or corrosion due to thedielectric effects of dissimilar metals. Many manufacturing situationsemploy adhesives to effect the bonding of two substrates, such asplastic to metal and metal to metal for reasons of economy andaesthetics.

Acrylate and methacrylate adhesives are generally known in the industry.U.S. Pat. No. 4,536,546 assigned to ITW of Glenview, Ill., the contentsof which are incorporated herein by reference, describes a methacrylatebased adhesive which employs a methacrylate ester monomer with aaldehyde-amine catalyst, a chlorinated polyethylene polymer and a graftcopolymer. U.S. Pat. Nos. 4,942,201 and 4,714,730, also assigned to ITW,the contents of which are incorporated herein by reference, describeadhesives made with methacrylate and acrylate based adhesives togetherwith various other additives and ingredients for use in the constructionof large objects.

Metal bonding adhesives are also described in the art. U.S. Pat. No.4,452,944, assigned to Lord Corp., describes a formulation composed ofacrylates and methacrylates monomers, polymerizable non-acrylic monomers(styrenes), isocyanate functional prepolymers reacted with polyols,phosphorous compounds, oxidation/reduction systems capable of effectinga room temperature cure, tertiary amines as accelerators, metalmolybdates and polybasic lead salts of phosphoric acid. The use ofurethane prepolymers and the absence of carboxylic acids in thisreference is intended to improve resistance to heat degradation of theadhesive.

In view of the above, it is apparent that progress in the development ofmetal bonding adhesives is an ongoing process. However, there is still acontinuing need for improved adhesive systems. Therefore, it is anobject of the present invention to provide an improved two-part adhesivesystem.

SUMMARY OF THE INVENTION

The present invention provides two-part adhesive systems that exhibitimproved onset of handling strength as well improved open times. In afirst embodiment, a two-part system is provided which includes anadhesive part and an activator part. The adhesive part is a mixtureformed from (I) an ethylenically unsaturated monomer, (II) a metalmolybdate, (III) a metal salt of an ethylenically unsaturated carboxylicacid in an amount less than 3 weight percent, and (IV) an ethylenicallyunsaturated carboxylic acid. The activator part includes a free radicalgenerator. Preferred ethylenically unsaturated monomers include acrylateor methacrylate ester monomers with methyl methacrylate beingparticularly preferred. The adhesive part can further include an admixedelastomeric material having a T_(g) less than −25° C. and soluble in theethylenically unsaturated monomer. Preferred elastomeric materials arepolychloroprene, polyacrylonitrile-butadiene copolymers, copolymers ofstyrene and isoprene, copolymers of styrene and butadiene, carboxylatedpolychloroprenes, carboxylated polyacrylonitrile-butadiene copolymers,copolymers of ethylene and vinyl acetate, copolymers of styrene andolefinically unsaturated hydrocarbons, polybutylene, acrylate-basedelastomers and mixtures thereof. One particularly preferred elastomer ispolychloroprene.

The adhesive part can further include additional components such anadmixed phosphorous-based adhesion promoter or an admixed core-shellimpact modifier swellable in the ethylenically unsaturated monomer. Apreferred phosphorous-based adhesion promoter is an acrylate ormethacrylate ester phosphate. Preferred impact modifiers are MBS impactmodifiers, ASA impact modifiers, ABS impact modifiers and mixturesthereof, with MBS being particularly preferred. The adhesive part canfurther include at least one admixed reducing agent.

The metal molybdates of the adhesive part preferably contain divalentmetals with zinc being particularly preferred. Likewise, the metalcations for metal salt of the ethylenically unsaturated carboxylic acidare preferably divalent metals. Preferred unsaturated carboxylic acidsfor the adhesive part are acrylic and methacrylic acid. One particularlypreferred metal salt of acrylic or methacrylic acid is zincdimethacrylate. More preferred amounts of the metal salt of theethylenically unsaturated carboxylic acid range from about 0.25 to about2.5 weight percent, with about 0.5 to about 2 weight percent being morepreferable.

The ratio of the adhesive part to the activator part is preferably fromabout 20:1 to about 1:1, with about 15:1 to about 4:1 being morepreferable. Free radical initiators for the activator part arepreferably peroxides, hydroperoxides, and mixtures thereof, with benzoylperoxide being particularly preferred. The activator part can furtherinclude an admixed epoxy resin such as a diglycidyl ether ofBisphenol-A. The activator part can also further include an admixedplasticizer.

In another embodiment, the present invention provides a curable adhesivecomposition including a mixture of an adhesive part and an activatorpart. The adhesive part includes (I) an ethylenically unsaturatedmonomer, (II) a metal molybdate, (III) a metal salt of an ethylenicallyunsaturated carboxylic acid in an amount less than 3 weight percent, and(IV) an ethylenically unsaturated carboxylic acid. The activator partincludes a free radical generator. Preferred embodiments are aspreviously described.

In another embodiment, the present invention provides a laminateincluding a first substrate and a second substrate bonded thereto with acured adhesive composition. The adhesive composition includes, in anuncured state, a mixture of an adhesive part and an activator part. Theadhesive part includes (I) an ethylenically unsaturated monomer, (II) ametal molybdate, (III) a metal salt of an ethylenically unsaturatedcarboxylic acid in an amount less than 3 weight percent, and (IV) anethylenically unsaturated carboxylic acid. The activator part includes afree radical generator. Preferred embodiments are as previouslydescribed.

The present invention also provides a method of preparing a laminatewhich includes contacting the surface of a first substrate with thesurface of a second substrate with an adhesive composition therebetween.The adhesive composition includes a mixture of an adhesive part and anactivator part, where the adhesive part includes (I) an ethylenicallyunsaturated monomer, (II) a metal molybdate, (III) a metal salt of anethylenically unsaturated carboxylic acid in an amount less than 3weight percent, and (IV) an ethylenically unsaturated carboxylic acid,and where the activator part includes a free radical generator.Preferred embodiments are as previously described. Substrates to bebonded are plastics, metals and combinations thereof. In an additionalpreferred embodiment, the surfaces of the substrates omit a primer priorto contacting.

In an alternative embodiment, the present invention provides a two-partadhesive system which includes an adhesive part being a mixture formedfrom (I) an ethylenically unsaturated monomer, (II) a metal molybdate,(III) an ethylenically unsaturated carboxylic acid, where the adhesivepart is substantially free of a metal salt of an ethylenicallyunsaturated carboxylic acid, and an activator part which includes a freeradical generator. Curable adhesive compositions, laminates, methods oflaminating using this alternative two-part adhesive system are alsoprovided.

Advantageously, the adhesives of the present invention provide animproved onset of handling strength while at the same time providingsuperior bond strength and fatigue resistance. These and otheradvantages of the present invention will become more apparent from thedescription set forth below.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plot graph illustrating the onset of handling strength as afunction of time for adhesive samples incorporating varying levels ofzinc dimethacrylate: (-▴-) 3.15 weight percent; (-★-) 2.5 weightpercent; (-▪-) 1 weight percent; and (-♦-) zero weight percent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a two-part adhesive system in whichadhesive compositions formed therefrom exhibit improved onset ofhandling strength in addition to improved bond strength and fatiguedurability. The first part of the adhesive system is an adhesive part isformed from a mixture which includes (I) an ethylenically unsaturatedmonomer, (II) a metal molybdate, (III) a metal salt of an ethylenicallyunsaturated carboxylic acid in an amount less than 3 weight percent, and(IV) an ethylenically unsaturated carboxylic acid, and optionally atleast one of the following components: (V) an elastomeric materialhaving a T_(g) less than −25° C. and soluble in the ethylenicallyunsaturated monomer; (VI) a phosphorous-based adhesion promoter, and(VII) a core-shell impact modifier swellable in the ethylenicallyunsaturated monomer. The second component is an activator part thatincludes a free radical generator. Advantageously, it has been foundthat the incorporation of less than 3 weight percent of a metal salt ofan ethylenically unsaturated carboxylic acid provides a faster onset ofhandling strength (i.e., a deceased fixture time) than is achieved withan amount greater than 3 weight percent.

Ethylenically unsaturated monomers to be used in accordance with theinvention are well known in the art. Such monomers are advantageouslyused in adhesive compositions due to their ability to undergo freeradical polymerization thereby cross-linking to result in a curedcomposition. Ethylenically unsaturated monomers to be used includeacrylate-based monomers, methacrylate-based monomers, non-acrylate basedmonomers or any combination thereof. The amount of ethylenicallyunsaturated monomers in the adhesive part is preferably at least about30 weight percent (wt. %). Generally, the amount preferably ranges fromabout 30 to 75 weight percent, with about 40 to about 60 wt. % beingmore preferred. The actual amount of the ethylenically unsaturatedmonomer is variable and can be easily ascertained by one skilled in theart.

Acrylate-based monomers (i.e., acrylate-based ester monomers) to be usedinclude, but are not limited to, polymerizable methacrylate esters andacrylate esters of the following general structures:

where “R” is a straight, branched, aromatic or cycloaliphatichydrocarbon radical having 1 to 20 carbons with a hydrocarbon radicalhaving 1 to 8 carbons being more preferred. In accordance with theinvention, the hydrocarbon radical can be substituted with or have oneor more carbon atoms replaced with a heteroatom (e.g., O, N, S).Likewise, the hydrocarbon radical can be unsaturated. Representativeexamples acrylate-based monomers include methyl methacrylate, ethylmethacrylate, butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexylmethacrylate, phenoxyethyl methacrylate, and tetrahydrofurfurylmethacrylate. More preferably, the acrylate-based ester monomer ismethyl methacrylate, ethyl methacrylate or combination thereof. Theacrylate-based ester monomers can also include inhibitors, such ashydroquinone, naphthaquinone, methylhydroquinone (MEHQ),tetramethylhydroquinone(TMHQ), tert-butyl catechol or combinationsthereof.

Non-acrylate based monomers to be used include aromatic and non-aromaticmonomers (or dimers) containing reactive vinyl groups. An example of anaromatic monomer to be is used is styrene.

Elastomeric materials to be optionally used in accordance with theinvention have a second order glass transition temperature (T_(g)) lessthan −25° C. and are at least dispersible (preferably soluble) in theethylenically unsaturated monomer. Preferably, the elastomers have aT_(g) less than −40° C. The elastomeric materials provide toughness tothe adhesive composition. Useful elastomers include synthetic highpolymers which exhibit plastic flow. Preferably, the elastomers to beused are supplied commercially as adhesive or cement grades. Elastomersuseful in this invention are described in detail in the “Handbook ofPlastics and Elastomers” pages 1-106-119, (1975) McGraw-Hill, Inc.,which is hereby incorporated by reference. A preferred group ofelastomers for use in accordance with the invention are polychloroprene(neoprene) and copolymers of butadiene or isoprene with styrene,acrylonitrile, acrylate esters, methacrylate esters, and the like.Additional useful elastomers are copolymers of ethylene and acrylateesters, homopolymers of epichlorohydrin and copolymers ofepichlorohydrin and ethylene.

Particularly useful elastomers for certain embodiments of the inventionare polychloroprene and block copolymers of styrene and butadiene orisoprene, such block copolymers being sold under the trademark Kraton®by Shell Oil Company. Block copolymers of styrene and diene monomers aredescribed in detail in U.S. Pat. Nos. 4,041,103 and 4,242,470 which areherein incorporated by reference.

Representative examples of useful elastomeric polymers, using theirletter designation according to ASTM D1418, their trade or common nameand chemical description are: CR-Neoprene-polychloroprene; NBR-Nitrilerubber-butadiene acrylonitrile copolymer containing about 25 to about 45weight percent acrylonitrile; COX-Hycar 1072-butadiene-acrylonitrilecopolymer modified with carboxylic groups; SBR-GR-S-styrene-butadienecopolymer containing about 10 to about 30 weight percent styrene;ABR-Acrylic rubber acrylate butadiene copolymer; and CO, ECO-Hydrin 100and 200-homopolymer or a copolymer of epichlorohydrin and ethyleneoxide. Additional useful elastomers are copolymers of ethylene andacrylate esters, such as methyl acrylate and ethyl acrylate, wherein thecopolymer preferably comprises at least 30 weight percent acrylateester, which elastomers are sold commercially by Dupont under the VAMACtrademark.

The amount of elastomeric material to be utilized in the adhesive partis variable and partially dependent on the type of elastomer andethylenically unsaturated monomer selected. These parameters can beeasily ascertained by one skilled in the art. Generally, the amount ofelastomer ranges from about 3 to about 35 wt. %, with about 6 to about20 wt. % being preferred, and about 8 to about 15 wt. % being morepreferred. One particularly preferred combination of elastomericmaterial and ethylenically unsaturated monomer is neoprene (e.g., DuPontNeoprene AD10) dispersed in methyl methacrylate. Thesedispersions/solutions are commonly known in the art aspolymer-in-monomer solutions. A description of making these type ofsolutions is found in U.S. Pat. No. 5,945,461, which is incorporatedherein by reference.

Phosphorous-based adhesion promoters to be optionally used in accordancewith the invention are well known in the art. The adhesion promoters areincorporated into the adhesive part to improve adhesion to metalsubstrates. Preferred adhesion promoters are phosphate esters derivedfrom phosphoric acid, which can include mono-, di-, or tri-esters.Hydroxyalkyl methacrylate or acrylate esters of phosphoric acid are morepreferred, such as hydroxyethyl methacrylate and hydroxypropylmethacrylate mon-, di- and tri-esters of phosphoric acid. Representativeexamples to be used include, but are not limited to,mono-methacryloxyethyl phosphate (mono-HEMA phosphate ester),bis-methacryloxyethyl phosphate (bis-HEMA phosphate ester),mono-methacryloxypropyl phosphate (mono-HPMA phosphate ester),bis-methacryloxypropyl phosphate (bis-HPMA phosphate ester), andcombinations thereof. One particularly preferred adhesion promoter ismethacryloxyethyl acid phosphate. In addition, the phosphate adhesionpromoter preferably contains at least about 90 wt. % mono- and di-estersin an equal amount which more preferably have an acid value betweenabout 250 and 310. Generally, the amount of the adhesion promoter in theadhesive part ranges from about 0.5 to about 10 wt. %, with about 1 toabout 5 wt. % being preferred, and about 2 to about 4 wt. % being morepreferred.

In accordance with the invention, a metal molybdate salt is incorporatedinto the adhesive part to inhibit corrosion and improve environmentalresistance. Preferred metals for the molybdate salt are divalent metals(e.g., group IIA and IIB metals) such as zinc, calcium, strontium,barium, among others. One particularly preferred metal molybdate salt iszinc molybdate. Metal molybdates are commercially available as aprecipitate on a carrier or binder (e.g., zinc oxide, calcium carbonate,magnesium silicate (talc)). One group of commercially available zincmolybdates are sold as pigments under the tradename Molywhite® fromSherman-Williams. One particularly preferred Molywhite® pigment isMolywhite 101. The metal molybdate is incorporated into the adhesivepart in a corrosion inhibiting amount. While the actual amount isvariable and partially dependent on the ratio of metal molybdate tobinder, the amount used generally ranges from about 0.1 to about 3 wt.%, with about 0.25 to about 1 wt. % being preferred, and about 0.4 toabout 0.8 wt. % being more preferred.

A core-shell impact modifiers to be optionally used in accordance withthe invention are well known in the art. The core-shell impact modifiersare swellable in the ethylenically unsaturated monomer (e.g., methylmethacrylate) but do not dissolve in the monomer. The impact modifiersare typically formed from graft copolymers. The core-shell graftcopolymers have a “rubbery” core, and an outer “hard” shell. The “core”or backbone polymer of the graft copolymers preferably has a glasstransition temperature substantially below ambient temperatures. The“shell” polymer which is grafted onto the backbone polymer preferablyhas a glass transition temperature substantially above ambienttemperatures. Ambient temperature in this context is defined as thetemperature range in which the adhesive is used. The impact modifiersprovide excellent impact strength in addition to non-sagcharacteristics, excellent thixotropic properties and improvedanti-sliding performance.

Examples of useful core-shell graft copolymers are those where “hard”monomers, such as styrene, acrylonitrile or methyl methacrylate, aregrafted onto a rubbery core made from polymers of “soft” or“elastomeric” monomers, such as butadiene or ethyl acrylate. Examples ofsuch core-shell copolymer to be used as impact modifiers are found inU.S. Pat. Nos. 3,984,497, 3,985,703, 4,034,013, 4,096,202, 4,304,709,4,942,201 and 5,112,691, which are incorporated herein by reference.Preferred core-shell impact modifiers are methacrylate-butadiene-styrene(MBS) graft copolymers, styrene-butadiene-styrene (SBS) graftcopolymers, and acrylate-styrene-acrylic acid (ASA) graft copolymers.Examples of those commercially available impact modifiers includePARALOID® BTA-753 from Rohm and Haas Company, KANE ACE B-564 from KanekaTexas Corporation and Geloy 1020 from GE Plastics.

As will be apparent to one skilled in the art, the amount of core-shellimpact modifier incorporated into the adhesive part should be sufficientto provide toughness to the adhesive and is thus variable. Generally,the amount ranges from about 5 to about 35 wt. %, with about 10 to about25 wt. % being preferred, and about 15 to about 20 wt. % being morepreferred.

The adhesive part of the invention also includes a metal salt of anethylenically unsaturated carboxylic acid to facilitate cross-linkingand curing of the adhesive. In accordance with invention, the metal salt(i.e., the metal ethylenically unsaturated carboxylate) is incorporatedinto the adhesive part in an amount less than about 3 wt. %.Surprisingly, it has been found that adhesives formed with less than 3wt. % metal salt of an ethylenically unsaturated carboxylic acid exhibitan improved (i.e., faster) onset of handling strength than achieved withidentical adhesives with greater than 3 wt. %. Preferably, the upperlimit of the carboxylic acid metal salt should be less than or equal toabout 2.5 wt. %, with about 2 wt. % or less being more preferred, andwith 1.5 wt. % or less being even more preferred. Preferably, the lowerlimit of carboxylic acid metal salt should be greater than or equal toabout 0.25 wt. %, with about 0.5 wt. % or greater being more preferred,and with 0.75 wt. % or greater being even more preferred. In addition,the above-described percentages for the metal salts refer to ex situcarboxylic acid metal salt incorporated into the adhesive part and notto metal carboxylates that can additionally form in situ upon mixing ofthe components.

As previously described, adhesives prepared with less than 3 wt. % exsitu metal carboxylates exhibit reduced fixture times as compared tocomparative adhesives prepared with greater than 3 wt. % ex situ metalcarboxylate. Preferably, the adhesives reach a handling strength of atleast 800 pounds per square inch (PSI) with at least about 10 percentreduction in time as compared to comparative adhesives as measured byASTM D1144. More preferably, the adhesives of the present inventionexhibit at least about a 20 percent reduction in fixture time ascompared to comparative adhesives.

Preferred metal cations for the metal ethylenically unsaturatedcarboxylates are divalent cations (e.g., group IIA and IIB metals) suchas zinc, calcium, strontium, barium, magnesium, among others. Aparticularly preferred metal cation is zinc. Preferred ethylenicallyunsaturated carboxyl anions for the carboxylates are derived fromethylenically unsaturated carboxylic acids such as those disclosed inU.S. Pat. Nos. 4,714,730 and 4,959,405, which are herein incorporated byreference. The ethylenically unsaturated carboxylic acids to be usedinclude monocarboxylic acids, polycarboxylic acids or combinationsthereof. Representative examples of these unsaturated carboxylic acidsare acrylic acid, methacrylic acid, maleic acid, fumaric acid,citraconic acid, mesaconic acid, acontic acid, traumatic acid, muconicacid, or combinations thereof, with acrylic and methacrylic acid beingparticularly preferred. Polycarboxylic acids (e.g., dicarboxylic acids)to be used with the invention can also be in the form of mono-esteralkyls of these acids. Preferably, the alkyl radical has 1 to 18carbons, with 1 to 6 carbons being more preferred. Unsaturatedmonocarboxylic acids also be in the form of hydroxy alkyl- (or hydroxyaryl-) acrylate and methacrylate acid esters of saturated di-andtri-carboxylic acids. Preferred hydroxy alkyl radicals have 1 to 10carbons, with 2 to 4 carbons being preferred. Preferred ethylenicallyunsaturated carboxylic acids salts are zinc, calcium, and magnesiumsalts of acrylic and methacrylic acid. One particularly preferred metalethylenically unsaturated carboxylate is zinc dimethacrylate.

In another embodiment, the adhesive part is prepared without theabove-described metal carboxylate salt (i.e., is substantially free of ametal carboxylate). The omission of the metal carboxylate has been foundto provide a surprising increase in working time as compared toadhesives where the adhesive part includes the above-described metalcarboxylate. In fact, adhesives prepared with an adhesive partsubstantially free of a metal carboxylate exhibit working times of atleast about 10 minutes.

In accordance with the invention, the adhesive part also includesethylenically unsaturated carboxylic acids such as those described abovefor the carboxyl anions. The ethylenically unsaturated carboxylic acidsto be used include monocarboxylic acids, polycarboxylic acids orcombinations thereof. In one embodiment, the ethylenically unsaturatedcarboxylic acids in the adhesive part have the same moiety as thecarboxylate anions for the metal salt. Preferred acids are unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,fumaric acid, citraconic acid, mesaconic acid, acontic acid, traumaticacid, muconic acid, or combinations thereof, with methacrylic acid beingparticularly preferred. The amount used generally ranges from about 1 toabout 10 wt. %, with about 3 to about 7 wt. % being preferred.

The adhesive part can further include one or more reducing agents.Reducing agents to be used are known in the art. Useful reducing agentsinclude dimethyl-p-toluidine, N,N-dimethylaniline, diethyl-p-toluidine,diethyl-m-toluidine, bis-propoxy-p-toluidine, N,N-diethylaniline,N-ethyl, N-hydroxy ethyl-m-toluidine, N,N-dimethyl-p-toluidine (DMPT),bis-hydroxyethyl-p-toluidine (HEPT), bis-hydroxyethyl-m-toluidine (HEMT)and the like. The amount of reducing agent is variable and partiallydependent on the type of activator used. The amount of reducing agent tobe incorporated into the adhesive part generally ranges from about 0.1to about 2 wt. %. These parameters can be easily ascertained by oneskilled in the art.

The activator part of the present invention includes at least a freeradical initiator to effect cross-linking upon mixing the adhesive andactivator parts. Free radical initiators to be used in accordance withthe invention are known in the art. Examples of initiators to be inaccordance with the invention include peroxides, hydroperoxides,peresters, peracids, azo compounds, radiant or UV energy and heat.Representative examples of peroxide and hydroperoxide compounds include,but are not limited to, benzoyl peroxide, cumene hydroperoxide, tertiarybutyl hydroperoxide, dicumyl peroxide, tertiary butyl peroxide acetate,tertiary butyl perbonzoate, and combinations thereof. Benzoyl peroxideis a particularly preferred free radical initiator. Likewise, heatenergy can additionally be applied to the adhesive to facilitate freeradical generation.

The free radical initiators are generally commercially available with acarrier in solid form or in liquid form (e.g., an epoxy resin orplasticizer) including solutions, suspensions, emulsions and pastes. Thecarrier can also includes additional excipients that are inert to theinitiators under normal storage conditions. Examples of excipientsinclude, but are not limited to, water, plasticizers, surfactant,thickening agents and so on. Extra amounts of those ingredients can beadded to adjust the concentration of the initiators in the activatorpart, or the viscosity and specific gravity of the activator part. Thecarrier should normally be less than 20% based on the total weight ofthe adhesive part and activator part, preferably less than 10%.

In a preferred embodiment, the activator part of the present inventionincludes a epoxy resin or polyepoxide as a carrier and a compatibilizer.Epoxy resins or polyepoxides to be used in accordance with the inventioncan be monomeric or polymeric, saturated or unsaturated, aliphatic,cycloaliphatic, aromatic or heterocyclic and they can be substituted, ifdesired, with other substituents besides epoxy groups, e.g., hydroxylgroups, ether radicals, halogen atoms, and the like. Epoxy resinssuitable in the practice of this invention include those disclosed inU.S. Pat. Nos. 2,500,600 and 2,324,483 which are incorporated herein byreference. One particularly preferred epoxy resin is a liquid diglycidylether of Bisphenol-A, which is commercially available from Shell OilCo., under the tradename EPON 828. The amount of the epoxy resin to beused is variable and generally in a 10:1 adhesive:activator system canrange from about 0.1 to about 50 wt. %, with about 10 to about 40 wt. %being preferred, and about 15 wt. % to about 25 wt. % being morepreferred.

Likewise, the activator part preferably includes a plasticizer toimprove the flexibility of the cured adhesive as well impact strengthunder temperatures below the elastomers T_(g). Examples of plasticizersthat can be used include phthalates, succinates, and adipates, suchexamples are dibutyl phthalate, diisobutyl phthalate, diisodecyladipate, butyl benzyl phthalate, Santicizer 278, dioctyl succinate, andmixtures thereof.

The activator part can further include the above-described impactmodifiers to provide additional impact strength to the adhesive of theinvention. The amount of impact modifier incorporated from about 5 toabout 25 wt. %, with about 10 to about 20 wt. % being preferred.

In accordance with the invention, both the adhesive and activator partscan further include thickening agents to alter viscosity, specificgravity and thixotropic properties. In one particular embodiment theadhesive part preferably, at room temperature, has a viscosity of about80,000 to about 110,000 centipose per second (cps). In anotherparticular embodiment, the activator part preferably, at roomtemperature, has a viscosity of about 50,000 to about 70,000 cps.

The adhesive and activator parts can also include free-radicalinhibitors/retarders to increase the shelf-life of the components and toimprove the curing profiles of the adhesives. These compounds act tosuppress the polymerization of monomers by reacting with the initiatingand propagating radicals and converting them either to non-radicalspecies or radicals of reactivity too slow to undergo propagation.Common inhibitors and retarders can be found in “Principles ofPolymerization” by George Odian, pp. 262-266, 3^(rd) edition, publishedby Wiley Interscience, incorporated by reference.

As previously described, the adhesives are advantageously provided in atwo-part system with an adhesive part and an activator part componentbeing separate prior to use. The two components are combined (i.e.,mixed together) just prior to use since the mixed composition has alimited working time (e.g., up to about 5 minutes at 25° C. for ahalf-inch bead). Preferably, the two components are sufficiently mixedto obtain a homogenous composition. The mixing of the two components areachieved by any method known in the art. The ratio of adhesive toactivator is variable and preferably ranges from about 20:1 to 1:1 byvolume, with about 15:1 to about 4:1 being more preferred. Oneparticularly preferred ratio to be used is about a 10:1 ratio by volumeof the adhesive:activator.

The present invention also provides an adhesive composition prepared bymixing the adhesive and activator parts of the above-described two-partsystem. As will be apparent to one skilled in the art, the adhesivecomposition is thus a reaction product prepared from the two componentsin which curing is effected by the cross-linking of the ethylenicallyunsaturated moieties. In addition to improved onset of handlingstrength, the adhesives exhibit good metal adhesion, peel strength,shear strength, dynamic fatigue, gap-filling ability, environmental andcorrosion durability, chemical resistance, and impact strength.

A particlarly preferred compositions of the adhesive and activatorportions of the two-part adhesive of the invention, are set forth belowin Tables 1 and 2.

TABLE 1 Composition of Adhesive Part Components: Acceptable Preferred(Weight Percent) Range Range Methyl Methacrylate (MMA) 30-75  40-60Chlorinated Rubber (chloroprene) 5-20  8-12 Hydroxyalkyl Phosphoric 1-5 2-3 Acid Ester Methacrylic Acid (MAA) 1-10 4-6 Zinc dimethacrylate 0-2.5 0.5-1.5 MBS core-shell 5-35 15-25 Zinc Molybdate on Binder0.2-4   0.3-0.8

TABLE 2 Composition of Activator Part Components: Acceptable Preferred(Weight Percent) Range Range Plasticizer 10-30  15-25 MBS core-shell5-25 10-20 Bisphenol-A Epoxy Resin 5-60 15-35 Peroxide Free RadicalGenerator 5-25 11-22

The adhesives of the invention are used to bond a variety of substratesfor various purposes. In fact, one advantageous use of the adhesives isin gap-filling applications that do not require anaerobic conditions.Substrates to be bonded with adhesives include those disclosed in U.S.Pat. No. 4,714,730 and include thermoplastics, thermosets, resins,composite resins, including polyester/glass, urethane/glass,epoxy/glass, epoxy graphite and epoxy-kevlar surfaces along with metalssuch as aluminum, steel (carbon and stainless), nickel, titanium andalloys thereof. The adhesives can be used to bond both primed andnon-primed surfaces. In fact, the adhesives of the present invention areparticularly useful for bonding non-primed substrates (e.g., substratesthat omit a primer). Typically, the surface of a first substrate iscontacted with the surface of a second substrate with the adhesive oneither or both surfaces.

In another embodiment of the invention, a laminate is provided whichincludes a first substrate bonded to a second substrate with a curedadhesive formed from the above-described two-part system.

The following non-limiting examples illustrate the improved propertiesof the adhesive systems of the present invention. All percentage are byweight unless noted otherwise.

EXAMPLES Stock Solutions and Non-ASTM Test Methods

Chloroprene-in-Methyl Methacrylate:

The chloroprene in methyl methacrylate (MMA) syrup was prepared byrolling 300 parts by weight of Dupont Neoprene AD10 and 1700 parts byweight of MMA in a glass bottle for about 2 days to obtain 15 wt. %polymer-in-monomer solution.

Zinc Molybdate Dispersion:

An 80 wt. % Molywhite 101 dispersion was prepared by high speeddispersing 80 parts by weight Molywhite 101 in 19.5 parts by weightdiisobutyl phthalate (DIBP), and 0.45 parts by weight BYK 1142(dispersant).

Sodium EDTA Solution:

A 5 wt. % Sodium EDTA solution was prepared by dissolving 20 parts byweight of Sodium EDTA in 190 parts by weight water and 190 parts byweight of 99% isopropanol.

Exotherm and Cure Analysis:

Peak exotherms were measured by placing 10 grams of adhesive in a 20 mlpolyethylene beaker to 10.0+0.5 grams. The 20 ml beaker was theninserted into a 100 ml polyethylene beaker containing ˜1 inch ofinsulatation on the bottom. The adhesive and activator were controlledto 25+0.5° C. for testing. A Type J thermocouple was placed in thecenter of material mass, and a notched cover for the probe width wasplaced over the insulating beaker. Peak temperature was recorded indegrees farenheight and time in minutes and nearest 15 seconds.

Comparative Example 1

A comparative adhesive formulation (sample 1) was prepared by mixing anadhesive part having the components listed in Table 3 with an activatorpart having the components listed in Table 4 in approximately a 10:1volumetric ratio of adhesive to activator.

Table 3 shows that the comparative adhesive (sample 1) reached ahandling strength of 800 pounds per square inch (PSI) for a 0.030 inchbead in 32 minutes as determined following ASTM D1144. Thus, the fixturetime for sample 1 was just over a half an hour. The development ofstrength for sample 1 is also shown in FIG. 1(-▴-).

Inventive Examples 2-8

Inventive adhesives (samples 2-8) were prepared as in Example 1 withdecreasing amounts of a group II metal methacrylate salt (zincdimethacrylate) to a certain the effect of lower concentrations onadhesive properties. The samples were prepared with less than 3 weightpercent ex situ zinc dimethacrylate: 2.5; 1; 0.8; and 0 wt. %. Thesample were evaluated in the same manner as sample 1. The formulationsand test data for samples 2-8 are shown in Table 3.

From Table 3, it is apparent that with decreasing concentrations of zincdimethacrylate a decrease in fixture time was observed. For example,comparative sample 1 with 3.15 wt. % zinc dimethacrylate exhibted afixture time of 32 minutes. Sample 2 which contained 2.5 wt. % zincdimethacrylate exhibited a fixture time of 29 minutes. Sample 3 whichcontained 1 wt. % zinc dimethacrylate exhibited a fixture time of 24minutes. Likewise, sample 8 which contained 0.8 wt. % zincdimethacrylate exhibited a fixture time of 25 minutes. The developmentof strength for samples 2 and 3 are also shown in FIG. 1 ((-★-) and(-▪-), respectively). Thus, the inventive samples exhibited reducedfixture times while exhibiting equivalent open times (4 to 5 minutes).This was considered surprising since the use of decreasing amounts ofzinc dimethacrylate would have normally been expected to result inincreased fixture times (e.g., greater than 32 minutes as exhibited bycomparative sample 1).

Moreover, sample 8 which was prepared with no zinc dimethacrylateexhibited an unexpected increase in working time by exhibiting an opentime of 10 minutes. This was double the working times of comparativesample 1 and inventive samples 2, 3 and 8. The development of strengthfor sample 8 is also shown in FIG. 1(-♦-)

TABLE 3 Test Value Method 1 2 3 4 Samples MMA Wt. % 4 15% Chloroprene inMMA Wt. % 68.90 69.50 71.00 70.00 syrup 1,4-Napthoquinone (NQ, Wt. %0.000012 0.000012 0.000012 0.000012 in ppm)  5% Sodium EDTA Wt. % 0.95 11 1 solution in 50/50 water/isopropanol N,N-dimethyl-para- Wt. % 0.7 0.70.7 toluidine (DMPT) Bis-hydroxyethyl-para- Wt. % 0.7 toluidine (HEPT)Bis-hydroxyethyl-meta- Wt. % toluidine (HEMT) Bis-methacryloxyethyl Wt.% 1.95 1.95 1.95 phosphate (P2M) Mono-methacryloxyethyl Wt. % 1.95phosphate (P1M) Methacrylic Acid (MAA) Wt. % 4.7 4.7 4.7 4.7 Zincdimethacrylate Wt. % 3.15 2.5 1 1 (Sartomer SR708) MBS core-shell Wt. %19 19 19 16 (Paraloid BTA-753) 80 wt. % Molywhite 101 Wt. % 0.65 0.650.65 0.65 ED dispersion Total Weight Wt. % 100.00 100.00 100.00 100.00Exotherm, time to peak Minutes: Plexus 12:15 12:45 12:00 27:00Temperature Seconds Exotherm, peak ° F. Plexus 267 255 267 285temperature Open Time Minutes ASTM 4 4½ 4 16 D1338 Time to 800 psi @Minutes ASTM 32 29 24 85 0.030″ (Fixture Time) D1144 Lap Shear UnexposedPSI ASTM 2000 — 2200 2215 Cold Rolled Steel D1002 (CRS) AdditionalProperties 14 days 5% salt fog CRS 1010 90% 103% 93% % retention 14 days5% salt fog Aluminum 45%  86% 70% % retention 6061-T6 14 days 5% saltfog Stainless 99% 103% 96% % retention Steel 304 T-peel CRS 1010Lb./linear 60 — 60 50 0.060″ ASTM D1875 inch Fatigue to AL 6061 50% of8,000,000 + {fraction (1/16)}″, ASTM D3613 ult. NF Strength Lap shearAluminum 2453 2453 2676 Unexposed 6061 Stainless 2183 2388 2580 steel304 Cold- 2132 2240 2215 rolled steel Lap shear Aluminum 1107 2112 187014 d. Salt spray 6061 24 hr. recovery Stainless 2157 2450 2482 steel 304Cold- 1923 2318 2029 rolled steel Lap shear Aluminum 700 1293 1380 31 d.Salt spray 6061 24 hr. recovery RT Stainless 2414 2353 2418 steel 304Cold- 1263 1945 1929 rolled steel Value 5 6 7 8 Samples MMA Wt. % 4 2.515% Chloroprene in MMA Wt. % 70.00 71.00 69.05 72.10 syrup1,4-Napthoquinone (NQ, Wt. % 0.000012 0.000012 0.000012 0.000012 in ppm) 5% Sodium EDTA Wt. % 1 1 0.09 0.9 solution in 50/50 water/isopropanolN,N-dimethyl-para- Wt. % 0.7 0.7 0.7 toluidine (DMPT)Bis-hydroxyethyl-para- Wt. % toluidine (HEPT) Bis-hydroxyethyl-meta- Wt.% 0.7 toluidine (HEMT) Bis-methacryloxyethyl Wt. % 1.95 2.5 1.95phosphate (P2M) Mono-methacryloxyethyl Wt. % 1.95 phosphate (P1M)Methacrylic Acid (MAA) Wt. % 4.7 4.7 4.7 4.7 Zinc dimethacrylate Wt. % 11 0.8 0 (Sartomer SR708) MBS core-shell Wt. % 16 19 19 19 (ParaloidBTA-753) 80 wt. % Molywhite 101 Wt. % 0.65 0.65 0.66 0.65 ED dispersionTotal Weight Wt. % 100.00 100.00 100.00 100.00 Exotherm, time to peakMinutes: 52:00 12:15 12:30 23:00 Temperature Seconds Exotherm, peak ° F.295 284 285 264 temperature Open Time Minutes 30 5 5 10 Time to 800 psi@ Minutes 100 26 25 44 0.030″ (Fixture Time) Lap Shear Unexposed PSI2220 — 2050 — Cold Rolled Steel (CRS) Additional Properties 14 days 5%salt fog CRS 1010 % retention 14 days 5% salt fog Aluminum % retention6061-T6 14 days 5% salt fog Stainless % retention Steel 304 T-peel CRS1010 Lb./linear 55 — 55 — 0.060″ ASTM D1875 inch Fatigue to AL 6061 50%of {fraction (1/16)}″, ASTM D3613 ult. Strength Lap shear AluminumUnexposed 6061 Stainless steel 304 Cold- rolled steel Lap shear Aluminum14 d. Salt spray 24 hr. recovery Stainless steel 304 Cold- rolled steelLap shear Aluminum 31 d. Salt spray 6061 24 hr. recovery RT Stainlesssteel 304 Cold- rolled steel

TABLE 4 Activator Component Supplier Wt. % Plasticizer CP Hall 18.25(diisodecyl adipate) Kraton G1652 Kraton Polymers 3.15 MBS Core-ShellRohm & Haas 17.20 (Paraloid BTA 753) Bisphenol-A Epoxy Performance 21.40(Epon 828) Polymers Benzoyl Peroxide Elf Atochem 40.00 55% Active(Luperco ANS) Total 100.00

Inventive Examples 9-11

Samples 9-11 were prepared to ascertain the utility of other elastomersin the adhesives of the invention. The adhesives were prepared in thesame manner as in the previous examples using a 10:1 volumetric ratio ofadhesive to activator. Samples 10 and 11 were prepared with equivalentamounts of commercially available elastomers (Nipol 1472 and Kraton1116, respectively) dispersed/dissolved in methyl methacrylate. Thecomponents and test data are shown in Table 5.

TABLE 5 Samples 9-11 Tradenames 9 10 11 MMA 8.849988 8.849988Carboxylated Chloroprene in MMA (25%) Neoprene AF 68.849988 Nipol 1472solution in MMA (25%) Nipol 1472 60 Kraton 1116 solution in MMA (25%)Kraton 1116 60 1,4-Napthoquinone NQ 0.000012 0.000012 0.000012 5% SodiumEDTA solution in 50/50 EDTA, sodium 0.9 0.9 0.9 water/isopropanol saltParafin wax, 30% solution in MMA Boler Wax 120- 3.3 3.3 3.3 155° F.Melting Point N,N-dimethyl-para-toluidine (DMPT) DMPT 0.7 0.7 0.7Bis-hydroxyethyl-para-toluidine (HEPT) HEPTBis-hydroxyethyl-meta-toluidine (HEMT) HEMT Bis-methacryloxyethylphosphate (P2M) P2M 2 2 2 Mono-methacryloxyethyl phosphate (P1M) P1MMethacrylic Acid (MAA) MAA 4.7 4.7 4.7 Zinc dimethacrylate (SartomerSR708) SR708 1 1 1 MBS core-shell (Paraloid BTA-753) Paraloid KM 75317.9 17.9 17.9 Molywhite 10 dispersion Molywhite 101 0.65 0.65 0.65Total Weight 100.00 100.00 100.00 Open Time, minutes @ 0.5″ Minutes 4.56.5 6 Exotherm peak time, min:sec Minutes:seconds 13:30 16:00 11:15Exotherm peak temperature, ° F. ° F. 256 255 267 Time to 800 psi @0.030″ Minutes 29 35 24 Lap shear, ASTM D1002, 0.05 Aluminum 6061 1948CF* 1912 CF 1692 CF inch/minute *CF—cohesive failure

Examples 12-13

Samples 12 and 13 were prepared to ascertain the utility of other zincmolybdates dispersions in lieu of Molywhite 101 as used in the previousexamples. Samples 12 and 13 were prepared with equivalent amounts ofMolywhite 212 and Molywhite 92 as shown in Table 6. As shown in Table 6,both inventive adhesives exhibited fixture times of less than 24 minuteswith 20 minutes for sample 12 and 22 minutes for sample 13.

TABLE 6 Samples 12-13 12 13 MMA 15% Chloroprene in MMA Dupont AD-1070.22  69.96 1,4-Napthoquinone (NQ, in ppm) 0.000012   0.000012  5%Sodium EDTA solution in 50/50 Aldrich Sodium EDTA 0.94   0.94water/isopropanol N,N-dimethyl-para-toluidine (DMPT) First Chemical DMPT0.7   0.7 Bis-hydroxyethyl-para-toluidine Cognic Emery 5710 (HEPT)Bis-hydroxyethyl-meta-toluidine Cognis Emery 5714 (HEMT)Bis-methacryloxyethyl phosphate (P2M) P2M 1.95   1.95 Methacrylic Acid(MAA) 4.7   4.7 Zinc dimethacrylate (Sartomer SR708) Sartomer SR708 1  1 MBS core-shell (Paraloid BTA-753) Paraloid BTA 753 18.5  18.5 70%Molywhite 212 in DIBP Sherman Williams 1.24 Molywhite 212 55% Molywhite92 in DIBP Molywhite 92   1.5 30% Parafin wax paste in MMA Boler Wax0.75   0.75 Total Weight 100  100 Open Time ASTM D1338 4.5   5 Exothermpeak time 10 gms, minutes 12:00 12:45 Exotherm peak temperature 10 gms,° F. 246  263 Time to 800 psi @ 0.030″ 20  22* * Shear ASTM D1144 Adh.Unexposed Cold Rolled Steel 1010 — — Time, PSI ASTM D 1002 min UnexposedAluminum 6061 ASTM D1002 2553 2501 5 0 10 3 15 35 20 308 24 1272 28 134332 1740

1. A two-part adhesive system with an improved onset of handlingstrength, comprising: (a) an adhesive part being a mixture formed from(I) an ethylenically unsaturated monomer selected from the groupconsisting of acrylate ester monomer, methacrylate ester monomer, ormixtures thereof, (II) a metal molybdate, (III) an ethylenicallyunsaturated carboxylic acid selected from the group consisting ofacrylic acid, methacrylic acid, or mixtures thereof, wherein saidadhesive part is substantially free of a metal salt of an ethylenicallyunsaturated carboxylic acid; and (b) an activator part which includes afree radical generator.
 2. The adhesive system of claim 1, wherein saidadhesive part further comprises an admixed elastomeric material having aT_(g) less than −25° C. and soluble in said ethylenically unsaturatedmonomer.
 3. The adhesive system of claim 2, wherein said elastomericmaterial is selected from the group consisting of polychloroprene,polyacrylonitrile-butadiene copolymers, copolymers of styrene andisoprene, copolymers of styrene and butadiene, carboxylatedpolychloroprenes, carboxylated polyacrylonitrile-butadiene copolymers,copolymers of ethylene and vinyl acetate, copolymers of styrene andolefinically unsaturated hydrocarbons, polybutylene, acrylate-basedelastomers and mixtures thereof.
 4. The adhesive system of claim 3,wherein said elastomeric material is polychloroprene.
 5. The adhesivesystem of claim 1, wherein said adhesive part further comprises anadmixed phosphorous-based adhesion promoter.
 6. The adhesive system ofclaim 5, wherein said phosphorous-based adhesion promoter is an acrylateor methacrylate ester phosphate.
 7. The adhesive system of claim 1,wherein said adhesive part further comprises an admixed core-shellimpact modifier swellable in said ethylenically unsaturated monomer. 8.The adhesive system of claim 7, wherein said core-shell impact modifieris selected from the group consisting of MBS impact modifiers, ASAimpact modifiers, ABS impact modifiers and mixtures thereof.
 9. Theadhesive system of claim 8, wherein said core shell impact modifier ismethacrylate-butadiene-styrene graft copolymer.
 10. The adhesive systemof claim 1, wherein said metal of said metal molybdate is a divalentmetal.
 11. The adhesive system of claim 10, wherein said divalent metalis zinc.
 12. The adhesive system of claim 1, wherein said adhesive partand said activator part are in a ratio from about 20:1 to about 1:1. 13.The adhesive system of claim 12, wherein said ratio is from about 15:1to about 4:1.
 14. The adhesive system of claim 1, wherein said freeradical generator is selected from the group consisting of peroxides,hydroperoxides, and mixtures thereof.
 15. The adhesive system of claim14, wherein said free radical generator is benzoyl peroxide.
 16. Theadhesive system of claim 1, wherein said adhesive part further comprisesat least one admixed reducing agent.
 17. The adhesive system of claim 1,wherein said activator part further comprises an admixed epoxy resin.18. The adhesive system of claim 17, wherein said epoxy resin is adiglycidyl ether of Bisphenol-A.
 19. The adhesive system of claim 1,wherein said activator part further comprises an admixed plasticizer.